The invention relates to heating furnaces and components thereof. More particularly, the invention relates to silicon carbide components for a semi-conductor diffusion furnace.
In the overall manufacturing process for the production of semi-conductor devices such as diodes and transistors, the most critical step is the part of the process referred to as doping. The doping process involves oxidation of thin silicon wafers for electrical insulation, followed by etching of cavities in the surface of the wafers, and the deposition of the dopant i.e. boron, phosphorus, arsenic, or antimony in the cavities which function as the transistor contact points. These semi-conductor components are made separately or in an integrated circuit array. The oxidation step and subsequent doping operation to which the silicon slices are subjected, involve a rapid heat and cool cycle in furnaces at temperatures in the range of from 1000 to 1350.degree.C. This critical heat treatment generally takes place in an electrically heated wire-wound muffle furnace. The silicon slices are placed on quartz or silicon kiln furniture which is then placed within the work tube of the muffle furnace and the silicon slices are fired through a predetermined time-temperature cycle. The diffusing agent is usually introduced in the form of a gas which is fed into the necked-down end of the diffusion furnace work tube. The gas then diffuses into the cavities which had previously been etched into the surface of the slices or wafers. In the oxidation and diffusion steps of the process the silicon slices are placed on boats or plates which are supported on appropriately designed kiln furniture. The kiln furniture and work tube or process tube as it is called, must be made of a material with excellent thermal shock resistance in order to permit rapid heating to, and rapid cooling from, temperatures in the order of 1000.degree. to 1350.degree.C. and back to room temperature. The material of which the kiln furniture and other furnace parts is constructed must also be of high mechanical strength, have the ability to retain its shape through a large number of heating and cooling cycles, and the material must not out gas i.e., introduce any undesirable impurities into the atmosphere of the kiln during firing operations, and the kiln furniture must not introduce any dust like contamination. Cleanliness and control of impurities are extremely important to the achievement of the ultimate desired electrical characteristics in the semi-conductor elements.
These demanding conditions severely limit the number of materials which can successfully be used to fabricate diffusion furnace parts or components. Generally the furnace components consist of a liner, a process tube which fits into the liner and which has a necked-down end for the introduction of the desired atmosphere, a paddle or wheeled carrier upon which are placed wafer supports, and the wafer supports or boats as they are known. These parts have been made of fused silica, however the silica components lose their mechanical strength and progressively devitrify with time at the processing temperatures involved which are generally above 1200.degree.C. In addition, silica components are very susceptible to extreme distortion from the frequent heating and cooling of the material, and will not withstand frequent cleaning with hydrofluoric acid which is normally required to maintain the necessary ultra pure furnace environment. In a more recent modification of the process, the diffusion liner i.e. the tube which surrounds the process tube, has been constructed of dense silicon carbide instead of the prior silica material, and used in conjunction with a silica process tube or inner tube. The silicon carbide possessed high thermal conductivity and high strength as compared to the silica. However, even the silicon carbide liners had their shortcomings. Although the structure of the material in the silicon carbide liner was dense, the liner overall was still not totally impervious to gases. Furthermore, because the structure was porous, the total surface area susceptible to contamination, was very large. As a result the ultra high purity requirements of the furnace environment were never completely attained and therefore the liner had to be used in conjunction with a silica process tube.
It is a principal object of the present invention to provide diffusion furnace components viz. a liner and/or process tube, paddle, and boat, which possess superior oxidation resistance, thermal shock resistance, high strength, the ability to retain their shape through a large number of heating and cooling cycles, an ultra high degree of chemical purity, impermeability to gases, and which have a very low surface area.